1. Field of the Invention
The present invention relates generally to a liquid crystal display device, and in particular to a bire-fringence control type liquid crystal display device such as an ECB (Electrically Controlled Birefringence) type or a DAP (Deformation of Aligned Phases) type.
2. Description of the Related Art
In a conventional birefringence control type liquid crystal display device, a pair of substrates each having transparent electrode and each having a surface orientation-treated in one direction are opposed to each other, and a liquid crystal material having a negative dielectric anisotropy is disposed between the substrates. Liquid crystal molecules are vertically aligned. Thus, a liquid crystal cell is formed. Polarizing plates are arranged on both sides of the liquid crystal cell.
In this liquid crystal display device, the liquid crystal presents a homeotropic texture state in which axes of liquid crystal molecules are substantially normal to the substrates when no voltage is applied to the electrodes. In this state, the direction of light incident in the liquid crystal display device coincides with the longitudinal axes (optical axes) of liquid crystal molecules; therefore, the liquid crystal cell does not exhibit birefringence effect. When the two polarizing plates are arranged such that the polarizing axes thereof are perpendicular to each other, the light entering the liquid crystal display device vertically is not subjected to the birefringence effect. Since the light cannot be transmitted through polarizing plate the display device is set in the dark state. On the other hand, when a voltage higher than a threshold is applied to the electrodes, the liquid crystal molecules are inclined at right angles with an electric field. Consequently, the liquid crystal cell exhibits birefringence effect and part of the incident light passes through the polarizing plate. Thus, the display device is set in the light state.
In order to attain uniform display characteristics, it is desirable that the directions of liquid crystal molecules be identical, and not at random, when voltage is applied to the electrodes. For this purpose, a previously processed homeotropic surface of the substrate is rubbed and, the liquid crystal molecules are inclined at a slight angle in a predetermined direction (i.e. a pretilt angle is provided).
In general, the liquid crystal display device is driven in a time-sharing manner ("multiplex driving") to reduce a number of electrical connection between a driving circuit and the liquid crystal display device. At the time of multiplex driving, some times birefringence control type LCD show an instability phenomenon of molecular alignment. The instability phenomenon is considered to be similar to a symptomatic phenomenon of a dynamic scattering (DS) effect. In this instability phenomenon, the liquid crystal molecules are inclined not only in the direction of the pretilt angle, but also in other directions. In addition, a reverse tilt portion occurs, and this portion moves with the passage of time. Consequently, the quality of displayed images is considerably degraded in the liquid crystal display device in which this instability phenomenon occurs. Specifically, the contrast is deteriorated, and non-uniformity occurs in the displayed images.